Cleaning bath and method of cleaning moving metal strip



H. L. LANE March 27, 1962 CLEANING BATH AND METHOD OF CLEANING MOVING METAL STRIP Filed Oct. 2, 1959 fi/My iNF ENTOR United States Patent Office 3,027,310 Patented Mar. 27, 1962 This invention relates to metal cleaning and particularly to continuously cleaning ferrous metal strip, and is adapted for descaling of steel or alloy strip. It also involves a ferrous metal cleaning salt bath composition.

This application is a continuation in part of application Serial No. 446,768, filed July 30, 1954, and now abancloned, which is a continuation-in-part of my application Serial No. 302,732, filed August 5, 1952, and now abandoned.

Modern methods of strip handling, for both plain and alloy steels, are such that the material must be cleaned at relatively rapid rates in a continuous process. It has been proposed to descale strip by various methods, many of which are based upon passing the strip through acid baths. These methods are merely adaptations of the old pickling baths used for sheet stock and the like and are not satisfactory for high speed continuous treating lines, particularly where alloy steels such as chromium containing steels are being processed.

An object of the present invention is to provide a method of continuously cleaning moving metal strip by electrolytic means in a salt solution;

Another object of the present invention is to make possible the continuous descaling of strip and alloy strip at high speed;

A further object of the invention is to provide a rapid method of continuously descaling chromium containing strip at high speed;

A still further object of the invention is to provide a method of continuous cleaning utilizing alkaline salt solutions;

Further objects and advantages of this invention will become apparent from a consideration of the following description of the process and the accompanying drawing illustrating an apparatus for carrying out the invention.

Referring to the drawing, a continuous metal strip 10, e.g. steel, is passed over an entering roll 11 into an electnolytic tank 12 containing an alkaline bath at elevated temperature (preferably 900 degrees F.). This alkaline bath is preferably of the following composition which, as shown, comprises a mixture of a major proportion of (alkali metal) sodium hydroxide, a substantial but minor or intermediate proportion of (alkali metal) sodium chloride, and a substantial but minor proportion of (alkali metal) sodium carbonate:

Other alkaline baths may, however, be used in the electrolyte tank 12, for example sodium sulphate may be used in place of sodium carbonate.

The strip is carried beneath the surface of the alkaline bath under guide rolls 13 and 14 which hold the strip beneath the surface of the alkaline bath. A pair of metallic electrodes 15 are placed on opposite sides .of the strip so that it passes between them in its travel between the two guide rolls 13 and 14. The electrodes 15 are impressed with a negative potential from any suitable source of direct current (not shown). Preferably the impressed potential is such that a current density of about 100 to 150 amperes per square foot is maintained on the strip.

The strip 10 is carried from the alkaline bath in tank 12 and through two asbestos covered squeegee rolls 16 which remove the excess alkaline materials from the strip and act to insulate the alkaline bath from the next succeeding baths.

From the squeegee rolls 16 the strip 10 is carried over a second entering roll 17 into a second electrolyte tank 18 containing an identical hot or molten salt bath alkaline in nature (preferably 900 F.). The hot salt solution in the second tank is of the same composition or comprises the same mixture as the molten alkaline salt bath in tank 12, having preferably the following composition:

Percent Sodium hydroxide About 82 Sodium chloride About 13 Sodium carbonate About 5 Other salt solutions may however be used, for example sodium sulphate may be substituted for sodium carbonate in the above composition. The strip is held beneath the surface of the bath by guide rolls 19 and 20. The elec trodes 21 are impressed with a positive potential from the direct current source (not shown). The strip is then carried out of the tank 18 between asbestos covered squeegee rolls 22 which remove excess salt and insulate the strip from the next succeeding rinse bath, which is preferably water.

In passing from the first alkaline bath to the second bath the strip 10 acts as a conductor between the baths which are otherwise insulated from one another. The process results in an evolution of gas, believed to be hydrogen, in the second tank with an accompanying reduction of scale and other forms of oxide. This action is particularly successful on chromium containing steel alloys which are ordinarily difficult to descale by the conventional processes.

The strip is carried from the squeegee rolls 22 beneath a guide roll 23 submerged in a rinse bath 24, preferably water, and then out of the rinse through rubber covered squeegee rolls 25 after which it is ready for further processing or coiling as the case may be.

The travel of the strip may vary depending upon its size and the amount of scale to be removed. Speeds in the neighborhood of 3 to 4 feet per minute to 50 feet per minute are generally satisfactory, though not limiting. The process may be applied to a variety of material e.g. hot or cold rolled steel strip and heat resisting steel strip.

While there is illustrate dand described a preferred embodiment and practice of this invention it will be understood that it may be otherwise embodied within the scope of the following claims.

What I claim is:

1. The method of continuously cleaning the surface of a ferrous strip comprising the steps of continuously and successively passing the strip through a molten alkaline bath wherein, the strip acts as the cathode of an electrical circuit, removing the strip from said alkaline bath, removing excess alkaline material from the strip, passing the strip into a second molten alkaline bath wherein the strip acts as the anode in the same electrical circuit, removing the strip fnom said second bath, said baths being insulated from one another whereby the strip acts as the current conductor between the two baths, and the two baths being made up of about 82% sodium hydroxide, about 13% sodium chloride, and about 5% sodium carbpnate.

2. The method of continuously cleaning the surface of a ferrous strip comprising the steps of continuously and successively passing the strip through a molten alkaline bath wherein, the strip acts as the cathode of an electrical circuit, removing the strip from said alkaline bath, removing excess alkaline material from the strip, passing the strip into a second molten alkaline bath wherein the References Cited in the file of this patent UNITED STATES PATENTS Kirschner Dec. 2, 1924 Reinhardt Feb. 5, 1935 Webster June 23, 1948 Murtland Mar. 11, 1958 FOREIGN PATENTS Great Britain Feb. 7, 1936 

1. THE METHOD OF CONTINUOUSLY CLEANING THE SURFACE OF A FERROUS STRIP COMPRISING THE STEPS OF CONTINUOUSLY AND SUCCESSIVELY PASSING THE STRIP THROUGH A MOLTEN ALKALINE BATH WHEREIN, THE STRIP ACTS AS THE CATHODE OF AN ELECTRICAL CIRCUIT, REMOVING THE STRIP FROM SAID ALKALINE BATH, REMOVING EXCESS ALKALINE MATERIAL FROM THE STRIP, PASSING THE STRIP INTO A SECOND MOLTEN ALKALINE BATH WHEREIN THE STRIP ACTS AS THE ANODE IN THE SAME ELECTRICAL CIRCUIT, REMOVING THE STRIP FROM SAID SECOND BATH, SAID BATHS BEING INSULATED FROM ONE ANOTHER WHEREBY THE STRIP ACTS AS THE CURRENT CONDUCTOR BETWEEN THE TWO BATHS, AND THE TWO BATHS BEING MADE UP OF ABOUT 82% SODIUM HYDROXIDE, ABOUT 13% SODIUM CHLORIDE, AND ABOUT 5% SODIUM CARBONATE. 